Compressional wave transducers



United States Patent-O 2,844,809 1 e r coMrREssIoNALwAvE TRANSDUCERS f I Laurence Batchelder, CambridgeQMass, assignor to Raytheon Manufacturing Company, Waltham, Mass., a

corporation of Delaware e Application nasal-Y5, 1955, Serial Nb. 479,907 9 Claims. (Cl. 340- This invention relatesto compressional wave transducers and more particularly .to transducers of the generaljtype disclosed in applicants U. S. Patent No. 2,408,028, issued September 24, 1946. g

--In, general, in such transducers, a plurality of piezoelectriccrystaljeleinents, orelectrostrictive ceramic elementsyaremounted on a metal plate from which they are separated by a sheet5oflinsulating material. Hereinaf t'er thefword crystal is used for hrevity,-but, whenever used, it is understood to include also ceramic ele-' ments, such as barium titanate, which have asuitable electroacoustic transducing property; 4 The crystals 'are arranged with their long axesfacing outward from the plate; 'Electrodes are applied to'op'posing faces of'the crystals to produce an electrical field in a direction at right angles to the long' axes. The array of crystals is covered by such a material as rubber'orother material that has the same transmission characteristics for compr'essional wave energy as seawater, and filled with va liquid, such as castor oil,.that. also has these characteristics,, and the array is mounted 'in' an opening in the hullbfthe vessel in which his tobe used. When an alternating M. F. is impressed on'thef electrodes, the crystals respond by alternately expanding andcontacting along their; longitudinal axis. .This motion causes the primary radiation .of compressional'waves fromthe, outer. V faces ofthe crystalswhich lie ina common plane; At

the same time -.that'.the long 'dimens'ionof the crystals is increasing orjdecreasi'ng', the other dimensions of each crystalfare; deereasing-lor increasing, respectively, to a' somewhat lesser degree inaccordance with Poissons ratio. This motion: causes radiation of compressional waves into the narrow spaces between the ,crystal blocks. The oil whichlfillsthese spaces canrespond only by flowing in andloutlpf the narrow crevicesand a large 'viscous dissipation of energy accompanies such restricted flow. One object of this invention is to provide an electroacoustic transducer ofjincreasedeificiency .by elfecting a substa'ntial reductionlin the amount of energy lost by viscous dissipation in such crevices. j u I p The. primary. radiation-arises at the end'faces of the individual crystal blocks, each of which vibrates forth and back likea piston. 7 .It is well known that a piston radiates sound most efiectively when it is surrounded by ast fi i 7 loading and decrease the. reactive loading which the medium imposes on .thepiston. The increased resistive loading permits an improved match of theradiation impedanc'e to the mechanical impedance of the crystal vibrators, with consequent increase-in the projector efli-' ciency. 1"Ideally,,the bafiie should be" infinitely stifi. For a loudspeaker designed to radiate into air, theideal'stifi- 'ness of approximated by any solid material,- such as wood. "The acoustic impedance of sea water or other liquid is so much higher than that of air that no material, even metal, is sufiicientlyis'tiif to 'constitute' an 1 afl sftsi a pr war e swea -14cc;

The baflle serves to increase the resistive Patented July 22, 1958 ond object of this invention is accomplished by providing each crystal block with a fictitious bafile which approaches'the ideal of infinite'stifiness. This fictitious baflle is produced by inserting a compliant reflector grid' of foam rubber, or a mixture of cork and neoprene. V

The electroacoustictransducer of this invention behaves in a reciprocal fashion and obeys substantially the electroacoustic reciprocity principle, which may be stated as follows: The quotient of the magnitude of the ratio of the open-circuit voltage at the output terminals (or short-circuit current) of the transducer, when used as a' sound receiver, to the free-fieldsound pressure referred to an arbitrarily selected reference point on or near the transducer, divided by the magnitude of the ratio of the sound pressure apparent at a distance from the reference 1 thetransmission and reception of compressional waves,

has the sameadvantages and objectiveswhen the trans ducer'is used for the reception of compressional waves. Other .and further-advantages of 'the invention will be apparent as the description thereof progresses, reference being had to .theaccompanying drawings wherein: Fig.;1 is a plan view of atransducer array without the pressurerelieving structure; i

--Fig. 2 is a sectionlalong the line 22 of 1 with i I the pressure relieving baifie added; 7

Fig. 3 is a plan view of the pressure relieving baflle;

- Fig. 4.is a section along theline 44 of Fig. 3;and ,Fig.- 5 is a diagram. of. the motion of a single erystal block greatly exaggerated. a j In Figs. 1 and 2, the transducer comprises an insulating disc 9Tbacked by a plate 10, and to which are attached piezoelectric crystals ll, which are held in position within an. annular flange -10a formed on the plate 10 by spacerslz through .18, 21 through 28, and 30 through. 34 which arefastened to' the plate 10. Oppos ing faces of the crystals. 11 are connected to'electrodes 36 through 38, 40 through 48, 50 through 58, 50a through 53a and 60. through 66. Electrodes 36, 40, 41, 44, 45,48 andStlaare connected to wire 67; electrodes 53a, 54, .57, 58, 62,63 and 66 to wire 68; electrodes 37, 38,42, 43, 46a, and 47rt0 wire 72; electrodes 55, 56, 60, 61, 64 and 65 to wire 71'.- Wire,72 is connected through-electrode 51-to cable 78. Wire 71 is connected 7 through electrode 52.tocable 78. Wire 67 is connected through electrode 51a. to cable 78a, wire 68 is connected through electrode 52a.lto cable 78a;Cables 78, and

78a, are passed to the-electrical part of the echo ranging system. through bushing 80, .and cable 78a through bushing 81. These connections are so made as to either excitethecrystalsin the proper phase or to gathertheir electrical outputs in the proper phase for use in theirest of the associated sonar apparatus, muchfas described in the applicants'cited patent. V ,Q

In addition, ajreflector assembly. 83, shown in Figs. 2, 3, and.4, i s mounted about the crystalsll. It comprises mutually perpendicular setsofstrips, one set 84 shown vertical'lyl in Fig. 3, and the other set 85 shown. horizontally inFig.' 3. The'sestrips are made. of a very.

compliantmaterial, such as foam 'rubber or. a mixture of neoprene andcork known as Qorprene These strips jare formed with notches 86 and 87 and arranged in an interlocked grid of the shape shown in Fig. 3 and bound together by a tape.88 about its outer edges. The strips are dimensioned to fit loosely between the crystals 11. The openings in the grid are large enough 'to receive a crystal 11 in each opening. The reflector grid'83 'fits down into the spaces between the crystals 11 and over the wires 67, 68, 71, 78 and 78a, and the electrodes 36 through 38, 40 through 48, 50 through 58, 50a through 53a, and 60 through 66, and extends up along the major axes of the crystals to within approximately a quarter wavelength at the operating frequency in the liquid of the radiating surfaces of the crystals. While this distance. of a quarter wavelength has been found to give the desired result, slight variations from this dimension may even produce better results. In use, the transducer is fitted'with a cover of rubber (not shown) and the space between the crystals and behind the rubber is filled with a liquid having transmission characteristics for compressional wave energy as close as possible to those of sea water. The assembled transducer is mounted in an opening in the hull of a ship with the crystals facing outward in much the same manner as is shown in Fig. 1 of applicants cited patent.

Fig. illustrates to an exaggerated degree the motion of a single crystal block. In its quiescent state it assumes the rectangular outline indicated by the heavy solid line 96. When the crystal is elongated longitudinally, it contracts laterally, as shown in the exaggerated form indicated by the dotted line 91. Similarly, when contracted longitudinally, it expands laterally as indicated by the dot-dash line 92. The amplitude of lateral motion increases progressively from a negligible amount at the outer end, and attains a maximum at a quarter wavelength hc/4, backalong the crystal ,at the propagation velocity in the crystal medium. In the design illustrated in Fig. 5, the total length of the crystal is a quarter wavelength Arr/4 at the propagating velocity in the crystal. While this is one common type of design, the invention is not limited to this type.

It happens that the velocity of sound in most crystal and ceramic materials is considerably greater (in the order of twice as great) than the velocityin castor oil, or whatever liquid is used to fill the transducer. Accordingly, a quarter wavelength in the oil extends from the outer end of the crystal block to some point, such as 93. It is obvious from Fig. 5 that the majority of the lateral displacement occurs at distances greater than a quarter wavelength in the liquid from the outer end of the block.

In accordance with one object of the invention, the inner segments of the crevices between crystal blocks are filled not with oil but with a pressure relieving material, as described above. Air cell rubber, or a mixture of neoprene and cork known as Corprene, has been found suitable for this purpose. This material extends from the base of the crystal blocks to a point approximately a quarter wavelength in oil from the outer end of the crystals. This pressure relieving material does not inhibit free lateral motion of the crystals, but it eliminates most of the oil which would otherwise be forced to flow in and out of the narrow crevices between the vibrating blocks. A major portion of the viscous losses is thereby eliminated.

The remaining portion of the crevices which is not filled with pressure release material has a dimension of one-quarter wavelength in the direction perpendicular to the common outer faceofthe crystal blocks. Acoustically, the crevices behave like a quarter-wavelength waveguide or transmission line for electromagnetic waves. This analogy is not exact because the side walls formed by the crystal blocks are not infinitely rigid, but it is sufiiciently close for practical purposes. At one edge of the quarter-wavelength oil-filled crevices, the pressure release material provides a termination which" is so compliantcompared with the oil that it hassubstantially zero impedance. This is analogous to a short circuit at the end of the quasi-equivalent, quarter-wavelength electric transmission line. By the familiar theory, it follows that, at the other end of the quarter-wavelength line or at the other edge of the crevice, the impedance is substantially infinite. Thus the crevices provide a fictitious stiff baffle at the common outer plane of the crystal blocks. The radiation from the outer faces of the crystals is therefore substantially the same as would be obtained from an array of pistons vibrating in an ideal stifi bafile.

While the relieving structure of the invention has been described as used with a particular transducer, this is merely by way of example, and the principle of the invention could be used with any transducer using an array of crystals. The important point is that a highly compliant relieving bafile be mounted at a position where it will have the described effects of relieving pressures built up by lateral oscillations, and of providing an effectively high acoustic impedance at the outer ends of the crystals.

" This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

'1. A compressional wave transducer comprising a plurality of rows of oscillatory elements mounted with their principal oscillatory surfaces in a common plane and their major axes perpendicular to this plane, and compliant material occupying thespace between the oscillatory elements extending from their bases up to a distance of approximately a quarter of a wavelength at the operating frequency from the principal oscillatory surfaces of said oscillatory elements such as to provide a minimum impedance at the inner portions of said oscillatory elements and provide a maximum impedance at the principal radiating surfaces.

2. A compressional wave transducer comprising a plurality of rows of oscillatory elements mounted with their principal oscillatory surfaces in a common plane and their major axes perpendicular to this plane, and comphant material occupying the space between the oscillatory elements extending from their bases up to a distance of approximately a quarter of a wavelength at the operating frequency from the principal oscillatory surfaces of said oscillatory elements such as to provide a minimum unpedance at the inner portions of said oscillatoryele'rnents.

3. A compressional Wave transducer comprising a plurality of rows of oscillatory elements mounted with their principal oscillatory surfaces in a common plane and their major axes perpendicular to this plane, and compliant material occupying the space between the oscillatory elements extending from their bases up to a distance of approximately a quarter of a wavelength at the operating frequency from the principal oscillatory surfaces of said oscillatory elements such as to provide a maximum impedance at the principal radiating surfaces.

4. A compressional wave transducer comprising a plurality of rows of oscillatory crystals mounted with their principal oscillatory surfaces in a common plane and their major axes perpendicular to this plane, and compliant material occupying the space between the crystals extending from their bases up to a distance of approximately a quarter of a wavelength at the operating frequency from the principal oscillatory surfaces of said crystals such as to provide a minimum impedance 'at the inner portions of said crystals and provide a maximum impedance at the principal radiating surfaces.

5. A compressional wave transducer comprising a plurality of rows of oscillatory elements mounted with their principal oscillabgry surfaces in a common plane and their major axes perpendicular to this plane, and compliant material comprising cork and neoprene occupying the space between the oscillatory elements extending from their bases up to a distance of approximately a quarter of a wavelength at the operating frequency from the principal oscillatory surfaces of said oscillatory elements such as to provide a minimum impedance at the inner portions of said oscillatory elements and provide a maximum impedance at the principal radiating surfaces.

6. A compressional wave transducer comprising a plurality of rows of oscillatory elements mounted with their principal oscillatory surfaces in a common plane and their major axes perpendicular to this plane, and compliant material comprising foam rubber occupying the space between the oscillating elements extending from their bases up to a distance of approximately a quarter of a wavelength at the operating frequency from the principal oscillating surfaces of said oscillatoryelements such as .to provide a minimum impedance at the inner portions of said oscillatory elements and provide a maximum impedance at the principal radiating surfaces.

7 7. A compressional wave transducer comprising a plurality of rows of oscillatory crystals mounted with their principal oscillatory surfaces in a common plane and their major axes perpendicular to this plane, and compliant material comprising cork and neoprene occupying the space between the crystals extending from their bases up to a distance of approximately a quarter of a Wave- 6 length at the operating frequency from the principal oscillatory surfaces of said crystals such as to provide a minimum impedance at the inner portions'of said crystals and provide a maximum impedance atthe principal radiating surfaces.

8. A compressional wave transducer comprising a plurality of rows of oscillatory crystals mounted with their principal oscillatory surfaces in a common plane and their major axes perpendicular to this plane, and compliant material comprising foam rubber occupying the space between the crystals extending from theirbases up to a distance of approximately a quarter of awavelength at the operating frequency from the principal t major axes perpendicular to this plane, and compliant material occupying the space between the oscillatory elements extending from their bases up to a distance of approximately a quarter wavelength at the, operating frequency from the principal oscillatory surfaces of said oscillatory elements.

No references cited.

STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2,844, 809

July 22, 1958 Laurence Batchelder It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line; 38, for "contacting" read contracting h cows lines 15 and 18, for "oscillating", each occurrence read oscilla or Signed and sealed vthis 23rd day of September 1953?.

ttest:

KARL H. AXLINE Attesting Ofiicer fiSEAL) ROBERT C. WATSON Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,844,809 July 22, 1958 Laurence Batchelder ied that error appears in the-printed specification It is herebfir certif ent requiring correction and that the said Letters of the above numbered pat Patent should read as corrected below.

"contacting" read -1.- contracting e column Column 1,. line 38-, for each occurrence, read oscillatory -m lines 15 and 18, for "oscillating'i,

Signed and sealed. this 23rd day of September 19523 SEAL) ttest:

KARL H. AXLINE Attesting Oflicer ROBERT C. WATSON Commissioner of Patents 

